1. Field of the Invention
The present invention relates to a light device synthesizing light from plural light sources to form synthesized light and emitting the synthesized light in the same direction, and to a projection image display unit including the light device such as laser scanning projectors.
2. Discussion of the Background
Recently, projection image display units (hereinafter referred to as “projectors”) using solid light sources such as LEDs and lasers are more developed, and they are expected to laptop projectors.
Japanese published unexamined applications Nos. 2006-189573 and 2001-154607 disclose small scanning projectors combined with three primary color lasers and a MEMS (Micro Electric Mechanical System) mirror, which are more developed because of being microminiaturizable.
A conventional scanning projector combined with three primary color lasers and a MEMS mirror is shown in FIG. 26. The projector in FIG. 26 includes laser diodes 1-R, 1-G and 1-B emitting laser beams of R (Red), G (Green) and B (Blue), respectively; lenses 2-R, 2-G and 2-B collecting laser beams from the laser diodes 1-R, 1-G and 1-B; dichroic mirrors 3-R, 3-G and 3-B reflecting only red light, green light and blue light, respectively and transmitting other light, respectively; a MEMS (Micro Electric Mechanical System) mirror device 501 equipped with a mirror having a variable inclined angle; and a control circuit 502 turning the mirror of the MEMS mirror device 501 in the horizontal and vertical directions and having the laser diodes 1-R, 1-G and 1-B emit intensity-modulated laser beams.
The control circuit 502 has a mirror controller and a modulator, and modulates the laser beam intensity to form an image on a screen 503.
Such color synthesizing methods need a collection lens for each laser and plural dichroic mirrors for synthesizing a light path for each laser beam, and therefore the number of parts increases, resulting in impairing downsizing and weight saving.
Japanese published unexamined applications Nos. 2007-121899 and 2007-333957 disclose methods of replacing plural dichroic mirrors with a sheet of diffraction element to synthesize a light path from the three light sources.
These methods use a sheet of diffraction element instead of plural dichroic mirrors in FIG. 26, which reduces the number of parts of the devices and downsizes the same.
International publication No. WO2005/073798 A1 discloses a configuration further locating the plural laser light sources on a mount as a package in addition to the above to reduce the number of parts of the devices, downsize the same and save weight thereof.
International publication No. WO2005/073798 A1 discloses a lighting device including three coherent light sources located on a same mount, emitting red, blue and green light; and a diffraction element diffracting the light emitted from the light sources so as to be a coaxial beam to multiplex each of the light.
Japanese published unexamined application No. 2002-207110 discloses a wedge prism synthesizing a light path of light from two light sources located on a mount, and an optical pickup device using the prism.
As mentioned above, the diffraction element can synthesize a light path of light from plural light sources with less number of parts. However, the diffraction element has the following problems.
Since the diffraction efficiency η typically varies according to wavelengths, it is not easy to prepare a diffraction element efficiently affecting plural wavelengths.
For example, a surface relief diffraction optical element is designed with a single wavelength. As shown in FIG. 27, when used for light having a wavelength different from the designed wavelength, the diffraction efficiency η lowers or a flare in imaging optics is caused thereby, resulting in deterioration of image quality.
A lighting device used for projector capable of synthesizing blue, green and red light and emitting the synthesized light is needed. They have a wavelength of from 0.4 to 0.48 μm, 0.5 to 0.55 μm and 0.6 to 0.7 μm, respectively.
Since they have large differences in wavelength, each of the blue, green and red wavelengths are difficult to have a diffraction efficiency η of nearly 100%, and it is difficult to produce only diffracted blue, green and red light having different orders each other.
Therefore, when such a diffraction element is used for a lighting device, the light use efficiency lowers and unnecessary light having no synthesized light path is produced, resulting in flare.
Alternatively, as disclosed in international publication No. WO2005/073798 A1, a volume hologram is thought to diffract only a desired wavelength. However, since the volume hologram has a very narrow scope of allowable wavelength having high diffraction efficiency, the diffraction efficiency lowers when the light sources have accidental wavelength errors, resulting in deterioration of light use efficiency and production of unnecessary flare light.
Particularly, a laser diode light source largely varies in wavelength according to the solid and the environmental variation such as temperature variation.
Thus, it is practically difficult to use the diffraction element for synthesizing a light path because of deterioration of light use efficiency and production of flare.
The wedge prism disclosed in Japanese published unexamined application No. 2002-207110 can synthesize a light path of light from two light sources located on a mount. A lighting device usable for laser scanning projector needs efficiently synthesizing light having different wavelengths from three (red, green and blue) light sources to form synthesized light and emitting the synthesized light to a pickup device.
Red and blue laser diodes are available, but a chip-shaped small light source such as a laser diode is not available as a green light source. Therefore, a green light source is difficult to locate close to other light sources and needs another configuration when used for a pickup device as a lighting device.
Because of these reasons, a need exists for a lighting device efficiently synthesizing light from three light sources and emitting the synthesized light, which is downsizable and improving light use efficiency.